OH, and did you already install longer parking brake cables?
With 3.5" of lift the parking brake is the limiting factor in many cases. Then the brake hose. Then the shocks and maybe even the driveshaft.
The modern 9, 10 and 11 leaf packs actually flex pretty decent, but if you are running out of spring travel at this point before other limiting factors, then yes, longer, flatter and wider springs can net you a little more. As long as you don't run out of shackle movement.
For moving the rear hanger back, you do run out of space towards the rear pretty quickly, but you could integrated it into a heavy duty bumper mounting brace probably.
There's plenty of room to extend the front hanger, but you don't necessarily want things too off-center to the point of having to re-drill the center pin hole too far off of the center of the leaf pack.
Some trucks do that from the factory, so there is something to it. Not sure what factors you have to take into account though, once you start doing that.
Paul